A virtual machine architecture logically partitions a physical machine, such that the underlying hardware of the machine is time-shared and appears as one or more independently operating virtual machines (VMs). A virtual machine monitor (VMM) creates the VM and runs on a computer to facilitate for other software the abstraction of one or more VMs. Each VM may function as a self-contained platform, running its own operating system (OS) and application software. The software running in a VM is collectively referred to herein as “guest software.”
The guest software expects to operate as if it were running on a dedicated computer rather than in a VM. That is, the guest software expects to control various events and have access to hardware resources on the computer (e.g., physical machine). The hardware resources of the physical machine may include one or more processors, resources resident on the processors (e.g., control registers, caches and others), memory (instructions residing in memory, e.g., descriptor tables), and other resources (e.g., input/output devices) that reside in the physical machine. The events may include interrupts, exceptions, platform events (e.g., initialization) (NIT) or system management interrupts (SMIs), and the like.
Hence, a VMM presents to other software (“guest software,” “guests” or simply “guest”) the abstraction of one or more VMs. The VMM can provide the same or different abstractions to the various guests. Each guest expects the full facilities of the hardware platform presented in the VM to be available for its use. For example, the guest expects to have access to all registers, caches, structures, I/O devices, memory and the like according to the architecture of the processor and platform presented in the VM. Further, each guest expects to handle various events, such as exceptions, interrupts and platform events (e.g., initialization) (INIT) and system management interrupts (SMIs).
Some of these resources and events are privileged because they are managed by the VMM to ensure proper operation of VMs and to protect the VMM and other VMs. For the privileged resources and events, the VMM facilitates functionality desired by guest software, while retaining ultimate control over these resources and events. The act of facilitating the functionality for the guest software may include a wide variety of activities on the part of the VMM. The activities of the VMM, as well as its characteristics, do not limit the scope of various embodiments described herein.